The brain includes gray matter, primarily collections of neurons which serve to process information and generate responsive signals, and white matter, primarily axons which serve to communicate these signals between gray matter regions and more distant parts of the nervous system. It is well known that specific regions within the gray matter are associated with particular functions; for example, motor skills are primarily controlled by regions in the cerebral cortex, the cerebellum, and the basal ganglia. Thus, there is significant interest in delivering signals to different regions of the brain to modify the brain's activity, and/or in measuring and interpreting signals from different regions of the brain to allow the brain to communicate with external devices. As an example, in the developing field of DBS (Deep Brain Stimulation), therapeutic (micro) electrodes are implanted within the brain to deliver timed impulses to desired nerve centers to treat a variety of disorders, in particular motor disorders such as Parkinson's disease and dystonia. As another example, in the developing field of man-machine interfaces, electrode arrays are implanted in the brain and the signals measured therefrom may be used to control prostheses, communication devices, or other machines. Further details can be found, for example, in U.S. Pat. No. 6,066,163 to John, U.S. Pat. No. 6,343,226 to Sunde et al., U.S. Pat. No. 6,354,299 to Fischell et al., U.S. Pat. No. 6,368,147 to Swanson, U.S. Pat. No. 6,454,774 to Flechenstein, U.S. Pat. No. 6,459,936 to Fischell et al., U.S. Pat. No. 6,463,328 to John, U.S. Pat. No. 6,484,059 to Gielen, U.S. Pat. No. 6,495,020 to Swanson, U.S. Pat. No. 6,539,263 to Schiff et al., U.S. Pat. No. 6,560,472 to Hill et al., U.S. Pat. No. 6,662,035 to Sochor, U.S. Pat. No. 6,587,724 to Mann, U.S. Pat. No. 6,731,986 to Mann, and U.S. Pat. No. 6,909,917 to Woods, as well as the patents cited in (and citing to) these patents.
Typically, electrodes for the foregoing applications are situated on one or more small-diameter leads implanted within a patient's head, with each lead bearing multiple electrodes (usually 4 to 8 electrodes). In appropriate circumstances, the same electrodes can be used for delivery of stimulation as well as sensing of intracerebral signals, as by switching selected electrodes between connection to stimulation delivery systems and connection to sensing systems. The leads are typically cylindrical, and bear internal conductors connected to annular electrodes spaced along the length of the lead. Because of the small diameter of the lead (and the limited space for conductor cables therein), the number of electrodes that can be placed on a lead is limited. Additionally, the annular design of the electrodes can make it difficult to determine the orientation of measured signals and/or to direct stimulation to desired brain regions (e.g., in the technique known as “field steering” or “current steering,” wherein different electrodes along a lead may take different roles as source electrodes and sink electrodes over time to determine optimal current paths for treatment). It would therefore be useful to have arrangements which allow electrodes to be placed along leads with greater density, and/or which allow better directional stimulation delivery and/or sensing ability.